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I've upgraded my Trek 920 Disc (2016) bike with Shimano Tiagra ST-4720 Dual Control Levers.

At the same time I've also replaced the disc brake calipers (Shimano BR-MT420) and the rear (Shimano Tiagra RD-4700-GS) and front derailleurs (Shimano Tiagra FD-4700-B).

Everything works well, except for the front derailleur which does not seem to move enough. Meaning that the chain either remains in the lower or higher gear and changing gear is - if possible at all - very rare.

In other words, I have the impression that the spacing between the two front chainrings (of the SRAM S1000, 42/28T crankset) is too big for what the front derailleur is capable of doing.

I suspect a compatibility issue between my front derailleur and the chainrings/crankset. I have tried to look up some specifications about the chainring spacing and the maximal movement of the front derailleur but without success.

Any suggestion on how to make this last piece of the puzzle working would be highly appreciated!

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The derailleur is designed for a FC4700 chainset, which from the specs below (x3-x2) is about 7.8mm. If the SRAM chainset is not wildly different from that then I don't suspect the spacing, and SRAM and Shimano chains and cogs are usually sympathetic.

The range of the front chainrings (42-28) is 14 teeth, within the 16 teeth gap the FD4700 can manage, but the 42 tooth is lower than most road derailleurs are specced for. The old road Tiagra (4600) was specced for 50 tooth minimum, but I can't find spec for 4700. 42 does seem like a bit of a stretch outside the minimum value.

So I don't suspect the gap between the two rings, but I do think the low tooth counts are a potential issue.

Beyond that, the newer Shimano front mechs are more involved in how you set them up, you need to eliminate user error in how you set it up. Particularly the limit screws.

  • Is the derailleur at the correct height and angle?
  • Did you set up the support plate?
  • Did you set up the converter correctly? https://si.shimano.com/pdfs/dm/DM-FD0002-05-ENG.pdf
  • Did you route the cable correctly?
  • Cable tension and limit screws? I would back off the limit screws to see if that helps, before setting them up again.

Extract of a table of Shimano crankset dimensions

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  • Thanks! Setting up the converter properly (as indicated in the manual provided in your link did solve my problem. It was 'ON' but definitely had to be 'OFF'. If I understand correctly changing it from 'ON' to 'OFF' increases the angle/distance the derailleur shifts for the same cable pull, right?
    – Fab
    Apr 13, 2020 at 15:39
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The SRAM S100 crank is really an MTB component, so I'm not surprised there are some shifting issues. Apart from chainring spacing the S1000 crank probably has a wider chainline than the Tiagra derailleur expects.

I think the best way to deal with this is to replace the crank with something that will work with the rest of the groupset. On this bike I would have thought that the Shimano GRX 10 speed groupset would be a much better fit than Tiagra. Interestly Tiagra 4700 and GRX 10 speed are compatible as they are both 10 speed groups that use the Shimano '11 speed' derailleur actuation ratios. Compatibility charts are here, look for 'FC-RX600-10'.

What you could do is replace the crank and front derailleur with the GRX items and be assured of compatibility. The GRX derailleur will work with the smaller rings on the crank and is compatible with the shifters.

The GRX crank comes with 46/30 rings so you would be losing the extra low ratios that the S1000 42/28 crank offers.

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